1. Field of the Invention
The present invention relates to a servo valve for controlling an internal combustion engine fuel injector.
2. Description of the Related Art
As is known, an injector comprises an injector body which defines a nozzle for injecting fuel into the engine, and houses a control rod movable along a respective axis to activate a pin closing the nozzle. The injector body also houses an electromagnetic control servo valve comprising a control chamber bounded axially on one side by the control rod, and on the other by an end wall having a calibrated axial outlet hole which, outside the control chamber, comes out axially inside a conical seat. The control servo valve also comprises a shutter, which engages the conical seat and is activated by an electromagnetic actuator to move axially to and from the seat to open and close the outlet hole and so vary the pressure inside the control chamber.
More specifically, the shutter is subjected on one side to the axial thrust exerted by the pressure of the fuel in the outlet hole, and, on the other side, to the action of the actuator and the axial thrust of a spring preloaded to keep the outlet hole closed when the actuator is not energized.
Known solutions as described above are unsatisfactory, on account of the characteristics and size of the shutter positioning spring having to be such as to exert a high preload, e.g. of about 70 newtons, to keep the outlet hole closed at high pressure (even as high as 1800 bars), so that powerful actuators are also required.
To minimize the above drawbacks, an attempt has been made to minimize the shutter sealing area to reduce pressure on the shutter. Because of the smaller sealing area, however, discharge of the control volume to activate the injection nozzle calls for relatively high lift (about 50 microns) of the shutter to uncover large enough flow sections, which lift is undesirable.
Moreover, a strong closing force of the shutter and direct axial exposure of the shutter to high pressure cause “bounce” of the shutter when closing. That is, the shutter actually bounces, as opposed to settling immediately, on the sealing seat.